Printed circuit boards are used in large numbers in telecommunications, computers and other electronic applications. Systems employing printed circuits normally use boards with circuits on both sides of one board, or in the case of multi-layer boards, circuits at each interface within a board. The boards are perforated with holes and the walls of the holes are made conductive to electrically connect circuits on one side of the board with the circuits on the other side of the board. The boards are generally made of paper-epoxy, paper-phenolic or epoxy-glass cloth. Initially, the perforations are non-conductive. However, the boards are typically catalyzed to make them receptive to electroless copper deposition over which electrolytic copper is plated. This results in the build-up of a layer of electrically conductive copper in the holes approximately 1-2 mils in thickness.
In U.S. Pat No. 3,769,179 owned by the assignee of the present invention, an acid copper sulfate electroplating bath is described which has the ability to deposit copper into and through holes in printed circuit boards, even when he holes are as small as 1/4th the thickness of the board. This "thru-hole" deposit is obtained from an electroplating bath containing between 70 and 150 g/l of CuSO.sub.4.5H.sub.2 O and between 175-300 g/l of H.sub.2 SO.sub.4. This bath is typically referred to as a high acid, low copper or HA-LC bath. The bath contains a small amount of a grain refining agent. One of the agents mentioned in the patent is instant coffee. Its use in a concentration of 0.1-1.0 g/l contributes to the production of a ductile deposit of copper in the perforations of the board as well as on the flat surfaces thereof. The electroplating bath is operated at temperatures between 20.degree. and 30.degree. C., preferably 22.degree. to 27.degree. C. and a cathode current density in the range of approximately 15-60 and preferably 20-35 amps per square foot.
The bath preferably contains between 1 and 10 cc per liter of 85% by volume phosphoric acid which serves to reduce burning of the deposit at high current densities while at the same time promoting uniform anode corrosion thereby contributing to the formation of a smooth electrodeposit. In addition, the bath contains between 10 and 250 parts per million of chloride ion which serves to prevent step plating, skip plating & tailing.
The patent states that instant coffee includes ground roasted and freeze dried coffee as well as the decaffeinated instant coffees. These coffees are marketed under a number of labels such as Maxim, Nescafe, Sunrise and Tasters Choice.
Because of its ready availability, relatively low cost, and ease of preparation, instant coffee has found widespread commercial acceptance as a grain refining agent in a high acid-low copper plating bath for printed circuit applications. However, the use of the instant coffee as a grain refining agent has not been entirely satisfactory in that its use in the plating bath has resulted in the formation of a gelatinous substance that tends to be codeposited with the copper on the substrate, resulting in a decrease in ductiliy and an increase in the tensile strength of the copper layer. The gelatinous substance appears to be related to the dispersant that is used in the manufacture of the instant coffee. The gel is difficult to remove from the plating bath by filtering because of its tendency to clog the filter medium.